In the oil and gas exploration and production industry, there are numerous downhole tools that feature radially extendable members. In the case of under-reamers, these members are in the form of blades or cutters that are extended once the under-reamer has passed beyond the end of the existing bore-lining casing, to allow the bore to be drilled beyond the casing to a larger diameter than the internal diameter of the casing. Once the reaming operation has been completed, the blades are retracted to allow the under-reamer, and the rest of the drill string, to be pulled out of the bore. An example of an under-reamer is described in applicant's International (PCT) Application Publication No. WO 00\31371, in the disclosure of which is incorporated herein by reference.
The blades of an under-reamer must be retained in a retracted configuration until the under-reamer has passed beyond the casing, to prevent damage to the casing. The blades may then released and extended. The means for retaining the blades in the retracted configuration should be reliable and secure, as premature extension of the blades is likely to cause significant damage that would be difficult and expensive to remedy. However, this must be balanced with the ability of the operator to release the blades when desired.
Furthermore, the inability, for any reason, to retract the blades of an under-reamer following completion of the under-reaming operation will make it difficult if not impossible to remove the under-reamer from the bore, as the under-reamer will not be able to pass into and through the existing casing. Remedying such a problem, if possible, involves considerable time and expense.
It is among the objectives of embodiments of the present invention to provide an under-reamer having a configuration that facilitates retraction of the under-reamer blades in the event of an operational difficulty.
It is among the objectives of further embodiments of the present invention to provide an under-reamer having a configuration that ensures reliable and secure retention of the under-reamer blades in a retracted configuration and reliable actuation of the blades to an extended configuration.
Other embodiments of the present invention relate to other forms of downhole tool featuring radially extendable members.